Power semiconductor devices, such as power diodes, power MOSFETs (Metal Oxide Field-Effect Transistors), power IGBTs (Insulated Gate Bipolar Transistors) or power thyristors, are designed to withstand high blocking voltages. Those power devices include a pn-junction that is formed between a p-doped semiconductor region and an n-doped semiconductor region. The device blocks—or is switched off—when the pn-junction is reverse biased. In this case, a depletion region (space charge region) propagates in the p-doped and n-doped regions. Usually one of these semiconductor regions is more lightly doped than the other one of the semiconductor regions, so that the depletion region mainly extends in the more lightly doped region, which mainly supports the voltage applied across the pn-junction. The semiconductor region supporting the blocking voltage is referred to as base region in a diode or thyristor, and is referred to as drift zone in an MOSFET or IGBT.
The ability of a pn-junction to support high voltages is limited by the avalanche breakdown phenomenon. As a voltage applied across a pn-junction increases, an electric field in the semiconductor regions forming the pn-junction increases. The electric field results in acceleration of mobile carriers present in the semiconductor region. An avalanche breakdown occurs when, due to the electric field, the charge carriers are accelerated such that they create electron-hole pairs by impact ionization. Charge carriers created by impact ionization create new charge carriers, so that there is a multiplication effect. At the onset of avalanche breakdown, a significant current flows across the pn-junction in the reverse direction. The voltage at which the avalanche breakdown sets in is referred to as breakdown voltage.
The electric field at which the avalanche breakdown sets in is referred to as critical electric field. The absolute value of the critical electric field is mainly dependent on the type of semiconductor material used to form the pn-junction, and is weakly dependent on the doping concentration of the more lightly doped semiconductor region.
The critical electric field is a theoretical value that is defined for a semiconductor region that has an infinite size in directions perpendicular to field strength vectors of the electric field. Power semiconductor components, however, have semiconductor bodies of finite size that are terminated by edge surfaces in lateral directions. For different reasons, such as imperfections of the crystal lattice at the edge surfaces, or geometrical effects leading to crowding of the electric field, the breakdown voltage of the component is lower in edge regions that are close to the edge surfaces than in inner regions that are distant to the edge surface. In order to compensate for the reduced breakdown voltage in the edge regions edge terminations are known that serve to reduce the electric held in edge regions as compared to the inner regions or at least reduce the effect of electric field crowding.
Different types of edge terminations are known, such as vertical edge terminations (mesa edge terminations), or beveled edge terminations. Beveled edge terminations have edge surfaces that are beveled.
Several methods for producing beveled edge terminations are known. Those methods include, for example, grinding, lapping, polishing or sandblasting an edge region of a round semiconductor body in order to form beveled edge surfaces. The quality of the resulting surfaces may be improved by implementing a subsequent etching process. However, a round semiconductor body may not be convenient for some types of semiconductor devices. Further, these methods either require that only one semiconductor device is integrated in a semiconductor wafer which is then processed in order to form the edge termination, or that the semiconductor wafer is subdivided into a plurality of semiconductor bodies (dies, chips) which are then processed in order to form the edge termination. However, handling small semiconductor bodies in one of the processes explained before may be difficult.
There is therefore a need to provide an improved method for forming a beveled edge termination of a semiconductor device.